1. Field of the Invention
The present invention generally relates to methods and apparatus for holding and positioning a workpiece mold into which metal parts are to be electroformed. More particularly, the present invention relates to a system for holding one or more micromolds within an electrolytic cell with the desired orientation, spacing, and electrical interconnections such that the features evinced in the micromold are reliably reproduced through an electroforming process.
2. Related Art
The LIGA process (from the German for Lithographie, Galvonoformung und Abformung) is a method of microfabrication based on deep x-ray lithography and electrodeposition for providing devices such as MEMS (micro-electromechanical systems). This process is capable of producing small metal parts having lateral dimensions of several centimeters, overall height dimensions of a millimeter or more, and feature sizes of less than a micrometer. To make such parts, a thick x-ray resist, usually poly(methyl methacrylate) (PMMA), is bonded or cast onto a substrate to which an electrically conductive layer has been applied. The PMMA resist is then exposed to synchrotron radiation through a patterned mask. The resist is subsequently developed, yielding a patterned mold attached to the substrate whose openings extend to the conductive layer. This mold is then filled by a process of electrodeposition to form the individual metal parts, the resist is chemically removed, and the finished parts are released from the substrate. This abbreviated process is illustrated in FIG. 1.
The manufacture of MEMS devices, therefore, requires the formation of metal structures having a wide range of feature sizes by electroforming them onto an electrically conductive metal layer placed onto one surface of a suitable substrate and which is exposed after the resist is exposed and developed. Because electroplating involves making electrical contact with the substrate surface upon which the electrically conductive material is to be deposited, it is important that the electrical contact with that surface be uniform and reliable as possible. Moreover, for optimum plating efficiency it is also important to be able to securely orient and hold that substrate in a fixed position while it is immersed into a electrolytic bath in order to insure uniform deposition.
The prior art is replete with examples of devices used to hold a round workpiece for electroplating. One of the first is U.S. Pat. No. 2,938,850 to Nali which provided an elongated wire frame to which ends in a set of spaced apart hooks into which the workpiece is held.
Another is U.S. Pat. No. 3,481,858 to Fromson which uses a vacuum supply to apply suction to a bell-shaped rubber cup for holding a wafer-like disk immersed in a plating bath.
Yet another example is U.S. Pat. No. 4,696,729 to Santini describing an electrolytic cell in which one of the containment walls contains a number of openings each having a diameter the size of the workpiece wafer and a lip extending into the opening to create an open aperture with a diameter slightly smaller than the diameter of the wafer. Wafers are held in place between the interior surface of the lip and a horizontal clamp and “o” ring assembly that pushes the wafer against lip.
An example of a “nested” plate assembly is described by U.S. Pat. No. 5,135,636 and shows a plate having a shallow cylindrical depression into which a wafer is held. Electrical contact to the plating surface of the wafer is made by means of three spring loaded contact “cams”.
U.S. Pat. No. 5,227,041, to Brogden, et al., teaches a dry contact electroplating structure. The structure includes a base member for immersion within an electroplating to solution. The base member has a central aperture defined by an aperture perimeter formed within the base member. A sealing ring is positioned adjacent to the aperture perimeter. The sealing ring forms a sealing connection with an object to be electroplated. A number of electrical contacts are positioned adjacent to the sealing ring. The electrical contacts form an electrical connection with one side of the object to be electroplated. A lid is positioned on the base member over one side of the central aperture. Thus, the lid protects the electrical contacts and one side of the object to be electroplated, while the other side of the object is exposed to the electroplating solution. Brogden, et al. further teach that the contacts preferably include relatively sharp tips for piercing any insulating substance which may be present on the wafer plating surface.
U.S. Pat. No. 5,228,967, to Crites, et al., teaches an electroplating system and method for electroplating wafers that includes supporting a plurality of wafers on a backing board in the electroplating tank such that one surface of each wafer is masked from the electrolytic reaction. Electrical contact is made by means of a beak-shaped pinch probe at a single point.
U.S. Pat. No. 5,312,532, to Andricacos, et al., teaches a multi-compartment electroplating system comprising a cathode-paddle-anode assembly for each compartment, wherein each of the assemblies has four supporting legs extending into the plating compartment attached to a cathode plate adapted for holding a wafer. The cathode plate further contains an aperture allowing access to the top surface of the wafer. Finally, a lifting jig is used to raise the wafer in place where it moves against an electrical contact wire on four sides around its perimeter.
U.S. Pat. No. 5,405,518, to Hsieh, et al., teaches an electrochemical etching apparatus containing a workpiece holder, which contains a first base plate and a second base plate to be joined together by screws; the first base plate having a recess on the front face thereof for receiving and retaining the workpiece, and both the first and second plates having a through hole to receive a contact electrode, which is electrically connected to a conductor wire. The assembly provides that the unintended portions of the workpiece, as well as the rear and side portions thereof, are protected from the etching fluid by first and second O-rings. Electrical contact, however, must be made from the rear of the workpiece.
U.S. Pat. No. 6,156,167, to Patton, et al., teaches an apparatus for electroplating a wafer surface that includes a cup having a central aperture defined by an inner perimeter, a compliant seal adjacent the inner perimeter, contacts adjacent the compliant seal and a cone attached to a rotatable spindle. The compliant seal forms a seal with the perimeter region of the wafer surface preventing plating solution from contaminating the wafer edge, wafer backside and the contacts. As a further measure to prevent contamination, the region behind the compliant seal is pressurized. By rotating the wafer during electroplating, bubble entrapment on the wafer surface is prevented.
However, the fixture described by Patton, et al. holds the wafer in an inverted position as do most if not all of the most recent examples of wafer holders used for electroplating. Moreover, electroplating requires immersing the wafer workpiece into the plating solution (i.e., a solution containing ions of the element being deposited). In those cases which require forming a MEMS structure by plating into a high aspect ratio mold it is necessary to orient and hold the mold in such a way as to avoid entrapping bubbles within the fine structure of the mold itself. This cannot be done easily or reliably using a device that inverts the substrate to which the mold is attached. Furthermore, when plating a plurality of MEMS structure across the surface of a large substrate, it is important to maintain close control the tolerance of the plating thickness as a percentage of the total thickness plated.
Furthermore, it will be appreciated that Brogden, et al. (U.S. Pat. No. 5,227,041), Patton, et al (U.S. Pat. No. 6,156,167), and others describe making electrical contact with a substrate (wafer) by means of one or several sharp metal tips. However, even with relatively sharp tips, one or more of the contacts may form a poor electrical connection with the wafer plating surface. This results in non-uniformity of the deposited electrically conductive layer and reduced yield since poorly plated parts must be discarded. What is needed also, therefore, is an easy and reliable method for making electrical contact to the substrate to be electroplated.